Once you have found a full source (online or print), use the CRAAP Test to determine if you should use it.
Currency
When was the information published or updated? Are the references to other sources up to date? Does currency matter for your topic?
Relevance/Coverage
Is this source relevant to your research question? Does the source meet the requirements of the assignment? Is the information too technical or too simple? Who is the intended audience? Does it add to your knowledge of the topic?
Authority
Who is the author? Is the author part of an educational institution or an organization? Can you find information about the author on the internet or with other resources? How often is this author cited? The author may be an individual or an organization.
Accuracy
Is this information correct and reliable? Are there spelling or grammar errors? Was the information reviewed or edited before it was published? What are other authors writing about the topic?
Purpose/Objectivity
What is the purpose of the information? Is it designed to sway your opinion? Does it project a bias? Are there other points of view presented?
The CRAAP test was creatd by California State University, Chico.
The top-level domain part of a web site's address can tell a lot about the legitimacy of the site
.edu - linked to an educational institution (though this domain can host personal web pages as well.)
.org - non-profit organizations or associations
.gov - a governmental department or agency, or government officials
.com - a commercial site, online service, or a for-profit organization
.mil - U.S. military organizations
.int- international organizations
.net - networking organizations
Look for citations, or some form of verification for the information presented on a web site.
The name and address of an author of a web site is reassuring, though this does not necessarily guarantee authority or legitimacy.